Plasma-wave measurements by Helios 1 show that the electric-field intensities in the solar wind are usually very low, much lower than those for comparable measurements near earth, where particles moving upstream from the bow shock often cause large disturbances in the solar wind. The most commonly occurring plasma wave detected by Helios is a sporadic emission at frequencies from about 1 to 10 kHz, between the electron and ion plasma frequencies. These waves are thought to be ion acoustic waves Doppler-shifted upward in frequency from below the ion plasma frequency. The maximum electric-field intensity of these waves is a few hundred microvolts per meter. At higher frequencies, from about 20 to 100 kHz, electron plasma oscillations are detected at frequencies near the local electron plasma frequency. These electron plasma oscillations are more intense, with field strengths sometimes as large as a few millivolts per meter, but occur very infrequently. Both the ion acoustic waves and the electron plasma oscillations show a tendency to occur at higher frequencies closer to the sun but no pronounced variation in intensity with radial distance from the sun. In four cases, electron plasma oscillations have been found in association with type III radio bursts.

Plasma wave electric fields in the solar wind: Initial results from Helios 1

ISEE 3 data form the basis for a study of the evolution of the electron plasma in the magnetotail lobe and plasma sheet regions as a function of radial distance from the earth. The data were plasma electron and magnetometer measurements taken when the ISEE 3 was tailward of the earth during three different orbits. About 91,000 electron spectra were obtained in terms of the magnetosheath, lobe, plasma sheet and radial downstream distances. The lobe density peaked at 60 earth radii, while the sheet density tapered very slowly downstream. At large downstream distances (about 180 radii), the plasma bulk flow velocity increased significantly, surpassing the solar wind velocity. The bimodal electron flow angle near the earth became a single tailward flow at 180 radii.

Evolution of the Earth's distant magnetotail: ISEE 3 electron plasma results

Geotail was launched on 24 July 1992 as the first new spacecraft of the International Solar Terrestrial Physics Program (ISTP). A primary objective of the Geotail mission is the acquisition of particles and fields measurements in the tail region of the magnetosphere. Some of the specific scientific objectives of the Geotail mission are; (1) assessment of the relative importance of solar-wind and ionospheric sources to the plasmas of the magnetotail, (2) a search for plasmoids and flux ropes, (3) direct observation of the effects of plasma acceleration and heating in the current sheet, and (4) measurements of currents and convective flows carried by magnetotail plasmas. The Comprehensive Plasma Instrumentation (CPI) aboard the Geotail spacecraft provides observations of the charged particles that comprise the plasma populations of the magnetosphere and the solar wind. In many cases the physical processes that contribute to the flow of energy and mass through the Solar-Terrestrial plasma system can be understood only through a detailed examination and analysis of the distribution of the particle velocities in the ambient plasmas. The CPI provides detailed measurements of these distributions. It employs three plasma analyzers: (1) the Hot Plasma analyzer (CPI-HP) for hot electrons and ions found in the plasma sheet and the inner magnetosphere; (2) a Solar Wind analyzer (CPI-SW) for cool plasmas with high bulk speeds such as those found in the solar wind and magnetosheath; and (3) an Ion Composition analyzer (CPI-IC) for identification of ion species such as H(+), He(+), He(++), and O(+). Along with measurements from other fields and particles instrumentation on Geotail, the CPI measurements provide a fundamental experimental base for investigations of solar-terrestrial plasmas and fields.

/pdf/the-comprehensive-plasma-instrumentation-cpi-for-the-geotail-3v2igjctgn.pdf

The Comprehensive Plasma Instrumentation(CPI) for the GEOTAIL Spacecraft.

Recent observational and theoretical studies of interplanetary shock waves associated with solar flares are reviewed. An attempt is made to outline the framework for the genesis, life and demise of these shocks. Thus, suggestions are made regarding their birth within the flare generation process, MHD wave propagation through the chromosphere and inner corona, and maturity to fully-developed coronal shock waves. Their subsequent propagation into the ambient interplanetary medium and disturbing effects within the solar wind are discussed within the context of theoretical and phenomenological models. The latter — based essentially on observations — are useful for a limited interpretation of shock geometric and kinematic characteristics. The former — upon which ultimate physical understanding depends — are used for clarification and classification of the shocks and their consequences within the solar wind. Classification of limiting cases of blast-produced shocks (as in an explosion) or longer lasting ejecta (or ‘piston’-driven shocks) will hopefully be combined with the study of the flare process itself. The theoretical approach, in spite of its contribution to clarification of various concepts, contains some fundamental limitations and requires further study. Numerical simulations, for example, depend upon a non-unique set of multi-parameter initial conditions at or near the Sun. Additionally, the subtle but important influence of magnetic fields upon energy transport processes within the solar wind has not been considered in the numerical simulation approach. Similarity solutions are limited to geometrical symmetries and have not exploited their potential beyond the special cases of the blast and the constant-velocity, piston-driven shock waves. These continuum fluid studies will probably require augmentation or even replacement by plasma kinetic theory in special situations when observations indicate the presence of anomalous transport processes. Presently, for example, efforts are directed toward identification of detailed shock structures (as in the case of Earth's bow shock) and of the disturbed solar wind (such as the piston). Further progress is expected with extensive in situ and remote monitoring of the solar wind over a wide range of heliographic radii, longitudes and latitudes.

Interplanetary shock waves generated by solar flares

We present a review of waves and instabilities in the solar wind, concentrating on those aspects that are likely to play important roles in influencing the dynamic and thermodynamic states of the general solar wind expansion. We consider in particular the roles played by various waves and instabilities in influencing the heating and expansion of the solar wind, the angular momentum of the solar wind, the solar wind thermal anisotropy, the heating and flow of alpha particles in the solar wind, the interstellar neutral particles that become ionized in the solar wind, and the ''fluidlike behavior'' of the solar wind. We include a brief review of the properties of the hydromagnetic wave modes, concentrating particularly on the Alfven mode, which has been observed to contribute significantly to the microscale fluctuations of the solar wind. But we also present a summary of observational evidence pertaining to the presence and action in the solar wind of waves and instabilities that are not among the hydromagnetic modes. (auth)

Waves and instabilities in the solar wind

Pioneer 8 wave and particle observations correlation showing broadband wave levels reduction during extended geomagnetic tail crossings

Pioneer 8 electric field measurements in the distant geomagnetic tail

We present Pioneer 8 observations of magnetosheath and interplanetary VLF electric fields, consisting of hourly ranges of the potential amplitude in a broadband channel (0.1 to 100 kHz) and in a 15% bandpass channel centered on 400 Hz. Significant signals are correlated with position with respect to earth, and with solar wind plasma parameters obtained from the lunar orbiting Explorer 35 spacecraft. We detect two principal features: noise bursts or spikes with duration less than approximately 10 sec, and persistent signals with durations typically 1 day or more. The noise bursts coincide with plasma and magnetic field discontinuities where these data are available for comparison. The persistent signals correlate loosely with solar wind density, and this correlation holds whether the density increases are due to interplanetary shocks, the ‘snow plow’ effect, or other processes. Although the experiment is too limited to provide unambiguous determination of the wave modes, at present it appears most likely that ion acoustic waves have been detected.

I. M. Green

Papers

Pioneer 8 electric field measurements in the distant geomagnetic tail

Very-low frequency electric fields in the interplanetary medium - Pioneer 8